#include <iostream>
#include <vector>


using namespace std;

int main()
{
	int N;
	cin >> N;
	int a, b;
	int ans=-10;
	for (size_t i = 0; i < N; i++)
	{
		cin >> a >> b;
		if (ans == -10)
		{
			if (b - a <= 0)
			{
				ans = -1;
			} else
			{
				ans = b - a;
			}
		} else
		{
			if (b - a != ans)
			{
				ans = -1;
				break;
			}
		}
	}
	cout << ans << endl;
	return 0;
}